Conventional circuits that use the current supplied by a photodiode are prone to noise as described in, for example "Divide and conquer noise in photodiode amplifiers", Electronic Design, Analog Applications Issue, Jun. 27, 1994 or in "Filtering Cuts Noise in Photodiode Amplifiers," Electronic Design, Analog Applications Issue, Nov. 7, 1994.
A log ratio amplifier or an operational amplifier based circuit can reduce noise in conventional circuits. See for example, SSM-2100, as described in "Analog IC Data Book" Volume 10, available from Precision Monolithic Inc., 1500 Space Park Drive, P.O. Box 28020, Santa Clara, Calif. 95052-8020. See also "New Op. Amp Ideas" by Robert J. Widlar (FIG. 21 "Opto-electric Pyrometer with Transmitter" at page 499), Linear Applications Databook, 1986, National Semiconductor Corporation, 2900 Semiconductor Drive, P.O. Box 58090, Santa Clara, Calif. 95052-8090. However, such circuits are expensive as compared to methods and apparatuses described below.
Use of analog dividers is complicated and expensive as seen from "Part One: Basic Operations," Nonlinear Circuits Handbook, 1976, published by Analog Devices, Inc., Norwood, Mass. 02062, USA. See also "Monolithic Op Amp--The Universal Linear Component" by Robert J. Widlar, (FIG. 10, Analog multiplier/divider at page 11) in Linear Applications Data Book referenced above.